Galvanic anode



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mTom/ey's B. DOUGLAS GALVANIC ANODE April 14, 1959 Original Fil edFeb. 1. .1955

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' Burke Doug/as BY WW United States Patent O 2,ss2,z13

GALVANIC ANODE Burke Douglas, Midland, Mich., assignor to The DowChemical Company, Midland, Micl1., a corporation of Michigan Originalapplication February 1, 1955, Serial No. 485,373. lgividog and thisapplication May 6, 1957, Serial No.

6 Claims. (Cl. 204-197) This invention relates to the cathodicprotection of metals and particularly to improved consumable anodes foruse in cathodic protection systems.

Cathodic protection systems are well-known in which a metal immersed inan electrolyte is protected from corrosion by means of a sacrificial orconsumable anode which is immersed in the electrolyte and iselectrically connected to the (cathodic) metal which is to be protected.

Sacrificial or consumable anodes comprise a metal which is anodic to themetal surface to be protected and some means, such as a metal corestrap, rod or cable to attach the anode to the surface to be protected.When the anode and the surface to be protected, or cathodic surface, arein an electrolyte and are electrically connected together, the resultingflow of current between the two electrodes greatly reduces the rate ofcorrosion of the cathodic surface.

The cathodic protection of pipe lines, ship hulls, metal sea walls, andwater tanks are examples of uses made of sacrificial anodes, such asmagnesium anodes, for ex ample. For many applications the expense ofreplacing anodes represents a substantial part of the cost of theprotective system. Further, in the case of anodes attached to shiphulls, for example, the anodes may be replaced conveniently only atinfrequent intervals, such as when the ship is in dry dock.

To avoid costly and frequent replacement of anodes, the use of largeanodes which have a long useful life has become common. Large anodes,however, tend to provide larger currents than are needed for theprotection of the cathodic surface and to that extent defeat theirpurpose of providing long-lived protection. Further, in marine servicethe larger anodes produce greater drag or resistance to the waterpassing the anodes than do smaller anodes and for this reason areundesirable for use on ships.

Resistance elements have been used in series with the electrical circuitbetween the anode and the cathodic surface to limit the current flowtherebetween. Resistance elements give only a partial answer to theproblem of extending anode life, however. Anodes are consumed as aresult of chemical attack as well as by current flow, and the resistanceelement does not affect the rate of chemical attack. Thus, anodecircuits including a resistance element operate at a lower efiiciency(measured in ampere hours per pound of anode) than do anodes which areoperated in systems not having series resistance elements includedtherein.

Anode efficiency, however, is but one criterion by which an anode isjudged. Another indication of the worth of a galvanic anode is itsthrowing power or, stated differently, the cathodic area in which theanode provides adequate cathodic protection. The throwing power ofanodes which are mounted close to the cathodic surface is quite limitedsince much of the current from the anode is used to protect areas of thecathodic surface which are closely adjacent to the anode. Thus,

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the close-in areas are over-protected (by excessive current) and remoteareas are inadequately protected. Resistor anodes, while limiting thetotal current, do not change the proportional distribution of theavailable current. 7

Another difiiculty encountered in connection with galvanic anodes isthat the anode consumption often follows a pattern such that themagnesium anode body (for example) becomes loosened from its mountingmeans or core while the body weight is still a significant percentage ofthe anode weight when installed. -For cathodic protection purposes, theanodic metal is wasted which remains after the electrical contactbetween the core or anode mounting means and the anode body is broken orbecomes a high resistance contact.

The labor cost for mounting anodes often represents a substantial partof the cost of the cathodic protection system, since men having severalbasic skills, such as welders, boiler makers, and painters may be usedin mounting the anodes on a ship hull, for example. An anode whichrequires workers having only a single skill or trade to mount wouldtherefore result in a saving in the cost of the, cathodic protectionsystem.

Difiiculties due to uneven consumption of the anode in different partsthereof have likewise been encountered a when one part of an anode iscloser to a cathodic surface than is another part of the anode. Acathodically protected water heater in which the magnesium anode rod isinserted from either the top or the bottom of the tank is an example ofan anode use in which the anode is subject to the above difficulty.

In such water heaters (or water softeners or similar tanks), the currentflow to the supporting wall of the tank, which is very close to theattached end of the anode, is often excessive. The result is that theattached end of the anode may be entirely consumed long before theremainder of the anode is depleted. This irregular depletion of theanode is undesirable because it will result in anode inefiiciency.

A principal object of this invention is to provide an improved galvanicanode which has a longer useful life per unit weight of anode material.

Another object of this invention is to provide an improved galvanicanode having improved current distribution to the cathode.

A further object of this invention is to provide an improved galvanicanode structure which needs no separate insulating means to separate theanode from the cathodic surface.

Yet another object of this invention is to provide an improved galvanicanode structure having means for selectively controlling the rate ofanode consumption in various parts of the anode body.

In accordance with the invention, there is provided a galvanic anodecomprising a consumable body portion and core means by which electricalconnection between the anode and a cathode surface may be made, theanode being enclosed in an insulating covering or casing. When the anodeis ready for use, the casing is provided with a plurality of aperturesin at least one surface thereof.

The invention, as well as additional objects and advantages thereof,will best be understood when the following detailed description is readin connection with the accompanying drawings, in which:

Fig. 1 is a plan view of a plastic coated anode in accordance with theinvention;

Fig. 2 is a side elevational view of the anode shown in Fig. 1;

Fig. 3 is a sectional view taken along the lines 33 of Fig. 1; v

Fig; 4 is a fragmentary sectional view, on an enlarged Patented Apr. 14,1959 scale, showing the plastic coating extending over the mountingstrap;

Fig. 5 is a sectional view taken along the lines 3-3 of Fig. l andshowing the anode consumption pattern after partial consumption of theanode shown in Fig. 1;

Fig. 6 illustrates an anode of the type shown in Fig. l as mounted onthe side of a ship hull;

Fig. 7 is an elevational view of a plastic coated anode in accordancewith the invention which is especially adapted for use with smallmetal-hulled craft and which requires no underwater connection to thehull of the craft;

Fig. 8 illustrates an anode in accordance with this invention which isadapted for use in water heaters or like uses;

Fig. 9 illustrates a cable-cored coated anode in accordance with theinvention;

Fig. 10 is a graph showing output current as a function of exposed anodearea;

Fig. 11 is a graph showing the percent of total anode current at variousdistances from. the anode under initial operating conditions;

Fig. 12 is a graph showing current density versus distance from anodefor three types of anodes under initial operating conditions;

Fig. 13 is a graph showing the current distribution (under initialconditions) of a perforated anode in accordance with this invention anda bare anode restricted with a resistor when the total current outputsof each anode are equal;

Fig. 14 is a graph showing current density versus distance from theanode for a bare anode and a coated and perforated anode underconditions of continued operation (i.e., after 24-30 hours ofoperation), and

Fig. 15 is a fragmentary sectional view showing an anode having alaminated or multi-layered covering.

Referring to Figs. 1 through 4 there is shown a galvanic anode indicatedgenerally by the numeral and comprising a consumable body 22, made ofmagnesium, for example, and having a metal core 24 (such as steel)embedded in the magnesium and bonded thereto and extending therefrom asa mounting strap 24a. The anode is encased in a plastic cover 26 whichfits closely about the anode body 22 and mounting straps 24a. The top ofthe plastic cover 26 contains a plurality of apertures 28 which areillustrated as being round although apertures of other configurationsmay be used. It should be noted that no apertures 28 appear in that partof the plastic coating which lies directly above the cores 24.

Referring particularly to Fig. 4, the plastic cover 26 extends along atleast a part of the mounting straps 24a. The mounting straps 24a usuallycontain a plurality of bores 30 which serve two purposes. The bores 30in the straps provide a convenient means by which the anode 20 may bebolted to a supporting structure. plastic cover 26' which extends oneach side of the straps 24a flows through at least one of the bores 30,causing the plastic on opposite sides of the strap to be bonded toitself. Such bonding of the cover 26 lessens the likelihood that theplastic cover 24 will tear off the mounting straps 24a and thence tearofl the anode body 22. In practice, the plastic cover 26 may entirelycover the mounting straps 24a, but may be cut back to the desired pointwhen the anode 20 is to be mounted.

The plastic cover 26 is a dispersion resin coating of dispersion gradepolyvinyl chloride resin or vinyl chloride copolymer resins plusplasticizer and stabilizer. Such anode coatings are electricallyinsulating and are physically tough. A coating thickness of $4; inch hasbeen found to be sufficient for most applications.

A specific coating formulation which may be used comprises:

60 parts by weight of polyvinyl chloride plastisol 40 parts by weight ofdi-octyl phthalate 1 part by weight of stabilizer such as Thermolite 3iAlso, the

\ 4 r which is an organo-tin chemical sold by the Metal and Thermit Co.of New York, NY.

5 parts by weight of mineral spirits such as Apco thinner Theplasticizer used is one which is relatively inelfective at roomtemperatures. However, when an anode which is heated to 350 to 406 F.,for example, is dipped into the dispersion, the heat increases theactivity of the plasticizer and a resinous coating is formed on the hotsurface of the anode.

While the coating of the general type described above proves verysatisfactory, other coatings may be used. For example a coating may beapplied by hot dipping an anode into an ethyl cellulose gel lacquer.Alternatively, the anode may be coated with neoprene which has anadvantage in that such a coating would be relatively immune to attack bylight hydrocarbons. Specifically neoprene coated galvanic anodes arewelladapted for use in cathodically protecting gasoline tanks orcompartments in tank ships.

As cast, dip coated anodes in accordance with this invention oftencontain a groove 31 in their top surface 32 which is disposed about inchfrom the peripheral edge of the anode body. The groove 31 is about Ainch wide and about inch deep. When the coating or cover 26 is appliedby dipping (or spraying, painting, etc.) the groove 31 becomes filledwith a rib 33 of the coating material. .During use, the rib 33 ofcoating which extends into the groove 31 lengthens the ion path throughthe electrolyte to the upper edge of the anode body 22. As the anodebody 22 is consumed, the peripheral surface portion of the body tends toremain as a shell to keep the plastic cover 26 tightly in place over theanode body 22. The result is a neater anode which has more resistance totearing of the cover 26 from the anode body 22 than does a partlyconsumed anode in which the sides of the anode body 22 are consumed andthe cover becomes loose.

One of the advantages of anodes made in accordance with this inventionis illustrated in Fig. 5. As the plastic. covered anode 20 is consumedduring use, the surface 32 of the anode which lies below the aperturesis primarily consumed, leaving relatively untouched the anode surfaceunder that part of the cover 26 where there are no apertures 28. Whilethe entire upper surface of the anode body 22 will eventually beconsumed, the surface area under the apertures 28 in the cover 26 willbe eaten away the fastest. The result is that the mag nesium anode body22 will remain firmly bonded to the cores 24 even though the distancebetween the surface 32 and the bottom 34 of the anode on either side ofthe cores 24 is less than the space between the core 24 and the bottomsurface 34 of the anode. In an anode 20 of the above type, almost theentire body weight of the anode gives useful cathodic protection.

Fig. 6 illustrates an anode 20 of the type shown in Fig. l as attachedto aships hull 36. The anode is attached by welding the ends of mountingstraps as at 38 to the hull.

Because anodes made in accordance with this invention require noadditional insulation device or electrical control to separate themelectrically from the cathodic surface, the cost of the anodeinstallation, particularly the labor cost, is materially reduced ascompared with the cost of installing a bare anode.

The theory concerning the operation of the anodes of this invention isnot fully understood. As has been observed, for an electrolyte of givenelectrical conductivity and assuming a cathodic surface to be protectedwhich is large with respect to the anode, the current output of theanode 20 may be controlled by regulating the number and size of theapertures, thus controlling the flow of current from the surface of theanode 20. The top surface of the anode is conveniently used for thispurpose. It is also known that the current output of various parts ofthe anode surface may be controlled by the size and humher of apertures28 above the various parts of the anode surface.

Data is given concerning operation of anodes of this invention underboth depolarized and polarized conditions. As the term is used herein,depolarized conditions are those in which the anode and cathode are eachat their natural potential while uncoupled in sea water. Under polarizedconditions, as the term is used herein, the anode is at its naturalpotential while the cathode is at a potential which is a function ofboth the current density and the length of time the current from theanode has been flowing to the cathode. In the tests made for the purposeof gathering data for the graph in Fig. 14, it was found that after24-60 hours the potentials tended to remain constant as if a steadystate condition of operation had been reached.

The graph shown in Fig. shows the relationship between the anode current(in milliamperes) and the exposed anode surface area (measured in squareinches of opening in the cover 26) for different types of anodes. Theterm exposed anode surface area as used herein refers to the area of theapertures in the coating. It is realized that in partly consumed anodes,however, the surface area exposed to electrolyte may be larger than theaperture area. The ion path to the cathode remains the same (i.e.,through the apertures). The coated anode data for the graph in Fig. 10were obtained by varying the number of inch diameter apertures 28 in thetop surface 32 of a coated anode. However, the current line 40 in Fig.10 is approximately the same when apertures 28 of other diameters (butof the same total area) are used.

As a practical matter, the aperture diameter is chosen depending upon atleast two factors. First, the strength. of the covering 26 of the anodeshould be maintained at a high value in order to lessen the tendency ofthe cover 26 to tear away from the anode body 22 when subjected toexternal forces. Secondly, the apertures 28 must be large enough topermit the anode corrosion product to be washed out of the casing. Theanode material may influence the characteristics of the corrosionproduct.

For anodes used in moving water, cover apertures 28 which are /2 inch indiameter have proven very satisfactory. The thickness of the cover 26may vary according to the material used and the installation of theanode, that is, whether the anode will be used in quiet or fast movingelectrolyte. In anodes coated with a polyvinyl chloride type coating athickness of .09 inch proved satisfactory for use in water moving at aspeed of 25 feet per second.

In tests of bare anodes and coated anodes placed in sea water under thesame operating conditions, it has been found that substantially nocalcareous coating builds up on the cathodic surface around a coatedanode mounted against a steel bulkhead in sea water while a substantialcalcareous coating is built up around an uncoated or bare anode.

While the existence of a calcareous coating is evidence of excessivecurrent, it is also evidence of paint damage and unsightliness. To theowner of small sea-going craft, the physical appearance of such craft isof prime importancc. Anode currents strong enough to cause calcareousdeposits cause the deterioration of the paint on that portion of thehull which is in the high current density area. The results have beenthat most ship owners have heretofore been reluctant to solve their hullcorrosion problems at the expense of having an unsightly hull or damagedpaint.

As is clearly seen from Figs. 11, 12 and 14 the current density on'thecathode close to the coated anodes is only a small fraction of thecurrent density on the cathode close to the bare anode under bothpolarized and depolarized conditions of operation. As shown in Figs. 11and 13, the use of a perforated coated anode results in a substantialsaving in close-in current over a resistor restricted bare anode ofequal current output.

Fig. 11 shows the percent of total current versus distance from theanode under depolarized conditions. It may be seen from this graph thatalthough coating the sides of the anode results in a considerable savingin current at distances close to the anode, the coated and perforatedanode of this invention results in a further and substantial reductionof the current to cathodic surfaces which are close to the anode. Bareanodes are even more wasteful of their current output.

The anode test setup used in securing the data for the graphs of Figs.11-14 comprised a series of concentric annular cathode segments made ofsteel with the test anode mounted in the center of the annular cathodesegments. The cathode segments were insulated from each other and eachhad a flat top surface approximately 3 inches wide (as measured radiallyfrom the center of the annulus). The test setup included 8 of suchcathode segments, mounted concentrically as stated above. When thegraphs of Figs. 11-14 inclusive are read, it should be remembered thatthe current density readings are actually for areas rather than forpoints as might be assumed from the distance scales on the graphs.

In the gathering of the data for Figs. 11-14, a steel cathode was usedand all the anodes used in the tests were identical in size and shape.The anodes were cylindrical in shape, having a diameter of 3 inches anda height of 4 inches, and were made of cell grade magnesium. One anodewas bare, one anode had its side coated and top bare, and one anode wasentirely coated and contained all the inch diameter perforations whichcould conveniently be made in the top surface thereof with inch spacingbetween adjacent apertures. The bottom surface of each of the anodes wascoated to protect that surface from the cathodic surface to which theanode was mounted. The measurements were made with the anodes immersedin sea water moving at a rate of approximately 6 ft./minute. In thosegraphs where current values are given, no attempt should be made torelate the currents to that current which is necessary to protect thecathodic surface. The experimental data shows only current distributionfrom the anode without regard to the current required to achieveprotection. Obviously, if more current is needed, larger anodes may beused.

Fig. 12 shows current density versus distance from anode underdepolarized conditions of operation. The graphs of Figs. 11 and 12 wereplotted using the same experimental data. It should be noted in Fig. 12that the current density close to the anode (3 inches from the anode) isabout 7 times as high for a bare anode as for a perforated and coatedanode, in which the perforations were placed as described above. Also, aperforated and coated anode puts a larger part of its output currentfarther away from the anode than does the bare anode. This largerfar-out current tends to increase the cathode area protected by a coatedanode as compared with bare anodes of similar current capabilities. Itshould be realized that in the graphs of Figs. 11 and 12 the totaloutput current of each of the anodes is different from the outputcurrent of the others.

The graph shown in Fig. 13, however, shows current distribution underdepolarized conditions in the case of equal total currents of a bareanode whose current output is restricted by a series resistor and aperforated coated anode. It should be noted that the close-in current ofthe perforated coated anode, that is, the current going to the cathodicsurface which is closely adjacent to the anode, is considerably lowerthan the close in current of the resistor-restricted anode. This graphindicates that the resistor in the resistor-restricted anode apparentlyrestricts only the total current output of the anode and has littleeffect on the current distribution pattern of the anode. Compared withFig. 11, the percent of current output curve for the bare anode is verysimilar to the corresponding curve for the resistor-restricted anode.Referring again to Fig. 13, it may be seen that in the case of thecoated and. perforated anode. only about 8 percent of the total currentwas used within 3 inches of the anode whereas about 23 percent of thetotal current of the resister-restricted anode is utilized in the samearea within 3 inches of the anode.

Considering the current distribution curve of Fig. 13, it appears to nowbe feasible to install anodes on surfaces which remain depolarized andstill achieve long lived protection. Previously on such continuallydepolarized surfaces, such as on a ships rudder, the anodes were quicklydepleted due to the excessive close-in current. Anode life of bareanodes in such installations was much less than the anode life of bareanodes when installed adjacent to polarized surfaces. Since ship anodesmay be installed conveniently only when the ship is in dry clock, theresult has been that ship hulls have not been protected as uniformly asdesired because of inability to replace anodes which were rapidlydepleted. The coated and perforated anode provides an answer to thisproblem by providing a controlled distribution of current and therebymaking possible more uniform protection of a ships hull or othercathodic surface.

A comparison is shown in Fig. 14 between a coated and perforated anodeand a bare anode of equal size regarding current density versus distanceunder polarized conditions of operation. Fig. 14 shows that the close-incurrent of the bare anode at 3 inches distance from the anode is about 4times the close-in current of a coated and perforated anode. Thus, evenunder polarized conditions of operation, the bare anode is quitewasteful in the usage of its current output in providing close-inprotection to a cathodic surface.

It should further be remembered that under conditions where the cathodicsurface is in rapidlymoving electrolyte, as for example on the hull of aship, the hull seldom becomes completely polarized.

Another practical advantage accrues to the use of coated and perforatedanodes which is not apparent from the graphs. Often the cathodic surfaceto be protected is coated, as by painting, and the coating effectivelyreduces the area of the exposed cathodic surface so far as anode currentrequirements are concerned. However, when high close-in currents occur,the coating peels off and an enlarged, bare cathodic surface ispresented to the anode. The bare surface in return requires more totalcurrent from the anode in order that the bare surface be adequatelyprotected. Because of the substantial reduction in close-in current whenanodes of this invention are used, the paint or coating on the cathodicsurface is relatively unaffected. It should be realized, however, that avery few brands of paint that were tested peeled off with even very lowcurrent to the cathodic surface.

To cite an example of the different current requirements for equal areasurfaces, a galvanized water tank may, and often does, require times theanode current for effective protection as does a glass lined water tankof equal size. A painted surface would not be as good as a glass linedsurface in reducing the current required for protection. However, it canbe appreciated that any loss of paint from the cathodic surface wouldincrease the anode current demand and, if effective protection is to bemaintained, would shorten the effective life of the anode. Thus, thefact that paint is not removed from the cathodic surface when coated andperforated anodes are installed results in further important advantagein extending the life of the anode.

The galvanic anode structures thus far described have been of the typewhich are bolted, welded, or otherwise fixedly attached to a structurewhich is to be cathodically protected. Referring to Fig. 7, there isshown a plastic coated galvanic anode 42 of generally cylindrical form.A metal mounting strap or cable 44 extends from the anode body and isbonded therein. The plastic coating or covering 46 of the anode isprovided with perforations 48 (apertures) in order to regulate the@Urlflli flow from the anode as previously described in connection withother coated and perforated anodes. The plastic coating 46, asillustrated, usually extends at least part way along the mounting strapor cable 44. The plastic coating or covering of the anode 42 alsoprovides an anode which is cleaner to handle than is a bare anode. It isanticipated that anodes of the type shown in Fig. 7 will find use asreadily demountable anodes for use on small vessels. In this type ofapplication, the anodes would normally be stowed away except when thevessel was anchored or tied up at a dock or pier. The anode strap wouldthen be fastened to a cleat mounted on the metal hull of the vesselcompleting the protective electrical circuit. Such an anode arrangementhas merit for small craft use, since many pleasure craft are docked oranchored far more hours than are operated. The demountable anodeprovides cathodic protection, yet may be removed easily so there is noextra drag in the water. The plastic covered anode is neater and moredesirable, from a housekeeping standpoint, than is a bare anode.Further, since galvanic anode surfaces become roughened during theirconsumption and often have small, sharp edges, the plastic coating overthe anode results in an anode assembly which is safer to handle than abare anode. The anode structure may be included as part of or combinedwith the fenders of the vessel if desired, thus eliminating a separateobject to be stowed while the vessel is in use. The covering 46 of theanode 42 need not be applied by dipping the anode body in liquidplastic, but may comprise a permanent casing in which bare anodes may bedisposed. Anodes and casings of this general type are described andclaimed in applicants copending application, Serial No. 485,438, filedFebruary 1, 1955.

While the advantages of the coated and perforated anodes of thisinvention have been described mainly in connection with ship hulls, suchanodes have many other applications.

For example, coated anodes having no perforations may be stock piled andstored without shelter from the weather for long periods of time withoutloss of anode weight by corrosion. Such anodes are provided withperforations of the required number and size at the time they are sold(or are to be used) for the particular installation in which they willbe utilized.

Difierent corrosion prevention applications require anodes of manydifferent varieties of performance characteristics. To stock all typesof anodes would place a considerable financial burden on a distributoror dealer. However, when a distributor stocks coated but unperforatedanodes in accordance with this invention, a minimum number of anodetypes and sizes serves to supply the anodes for a wide variety ofapplication situations when the anodes are given the requiredperforation pattern.

The availability of an unperforated coated anode is also attractive tocorrosion engineers who prefer to maintain some degree of secrecy abouttheir ideas as to what is the best solution to a particular corrosionproblem.

Coated and perforated anodes may be used in many applications whereresistor-restricted anodes or bare anodes are now used. Pipe linecathodic protection systems can make use of the anodes of this inventionby mounting the anodes closer to the line than heretofore has beenpractical because of the excessive local current to the nearby pipe.

The use of a coated and perforated anode in a water tank is illustratedin Fig. 8. The anode, indicated generally by the numeral 50, extendsupwardly from the bottom of the tank 52, but could be mounted to extenddownward from the top of the tank. In either method of mounting,excessive current usually flows to the mounting end of the tank becausethe surface 54 from which the anode 50 is mounted is closer to themounting end of the anode 50 than are the sides of the tank 52. Thus,

9 the lower end of the coating 56 on the anode 50 in the tank hassmaller apertures 58 than appear in the coating over the remainder ofthe anode. The smaller apertures 58 near the bottom end of the anoderestrict the current which flows from that part of the anode, thuscausing a more uniform expenditure of the anode than would occur if theanode apertures were all uniform in size. Such selective current flowfrom the anode 50 would not be possible in a resistor-restricted anode.In addition, the coating 56 provides physical support for the anode 50.

Fig. 9 illustrates a cable-core coated and perforated anode, indicatedgenerally by the numeral 60, made in accordance with this invention. Theanode coating 62 extends along the mounting cable 64 for a considerabledistance from the anode body 66 to prevent strong local current flowbetween the anode and the nearby mounting cable 64. Such cable anodesare well adapted to be clamped to a submersible metallic net, forexample. The size and number of perforations 68 of the anode may bevaried to accommodate a wide variety of corrosion control situations.

Any of the anode assemblies described above may be coated with ananti-fouling coating in addition to or as a substitute for thepreviously described coatings. An anti-fouling coating which is suitablefor use on anodes for use in sea water is a copper salt of polyacrylicacid. Such coatings can be made relatively insoluble by controlling thedegree of polymerization of the coating material. The coating isphysical tough and has a double anti-fouling action. The copper is toxicto small marine oragnisms and since the surface of the coating dissolvesat a slow rate, the organisms cannot readily adhere to the coating.

As a precaution against baring the anode due to the dissolving of theanti-fouling coating, or because of the electrical properties of somecoatings it is sometimes advisable to provide an under coating ofnormally insoluble material (such as described previously herein, forexample), prior to applying the anti-fouling coating.

Another anti-fouling coating may be provided by dispersing copperparticles through the non-soluble coating materials.

Another use of slowly soluble coating materials 1s to provide a delayedaction anode for use in installations where mounting the anodes isexpensive or can be done only at infrequent intervals. Thus, animmediately operative anode and a delayed action anode may be installedtogether with a coating on the delay action anode which is calculated toexpose the galvanic metal of that anode at approximately the time of theend of the useful life of the immediately operative anode.

Fig. illustrates an anode 22 having a coating consisting of two layersor laminations 26a, 26b. The layer 26a, which, as illustrated, containsone or more apertures 28, is the less soluble of the two layers. Theouter layer 26b is the more soluble layer, covers the inner layer 26aand fills in the aperture or apertures 28.

It is now customary to attempt to achieve long-lived cathodic protectionby the use of larger and larger anodes. Larger anodes, while providingprotection over a longer period than do smaller anodes, have highercurrent flow and thus are ineflicient. Two small anodes, one of them adelayed-action anode, may be used to provide continuous protectionwithout the inefliciency of a single large anode.

Among the suitable coatings which will slowly dissolve are polyacrylicacid (or salts thereof), polyvinyl alcohol, methyl cellulose plusenzyme, or natural gums plus bacteria. Arayakraya is an example of anatural gum which may be used. It is relaized that not all the abovecoating materials are suitable for use in mobile installations such asships. However, many stationary or seldom moved metals in sea-water orother saline electrolyte may be given long range protection by suchdelayed action anodes.

Delayed action anodes may likewise be used in tanks which are used tostore or transport petroleum products or other materials. The anodecoating for such delayed action use is chosen from substances which havelow solubility rates in the electrolyte to which they will be exposed.

Thus, it is apparent that the present invention provides improvedgalvanic anodes which have longer life, have better currentdistribution, are easier to store, and are more adaptable to a widevariety of corrosion control situations than are conventional bareanodes or resistorrestricted anodes.

This is a division of my copending application, Serial No. 485,373,filed February 1, 1955.

I claim:

1. An article for use in the cathodic protection of metal surfacesimmersed in an electrolyte, comprising a galvanic anode assemblyincluding a galvanic anode body, a metal core bonded into said body, anda closely fitting, electrically insulating covering surrounding saidanode body, said covering being a laminated covering having an innerlayer and an outer layer, said covering being soluble in saidelectrolyte, the inner layer being substantially less soluble in theelectrolyte than is the outer layer, the outer layer entirely coveringsaid anode body.

2. An article in accordance with claim 1, wherein said inner layer iscomposed of vinyl dispersion resin and said outer layer is composed ofpolyacrylic acid.

3. An article in accordance with claim 1, wherein said inner layer iscomposed of vinyl dispersion resin and said outer layer is composed ofpolyvinyl alcohol.

4. An article in accordance with claim 1, wherein said inner layercontains a plurality of apertures.

5. An article in accordance with claim 4, wherein each of said apertureshas an area exceeding the area of a circle having a diameter ofone-eighth inch.

6. An article for use in the cathodic protection of a metal surfaceimmersed in an electrolyte, comprising a galvanic anode assemblyincluding a galvanic anode body, a metal core bonded into said body, anda closely fitting, electrically insulating covering surrounding saidbody, said covering being a laminated covering having an inner layer ofvinyl dispersion resin and an outer layer composed of a copper salt ofpolyacrylic acid.

References Cited in the file of this patent UNITED STATES PATENTS915,846 Friedheim Mar. 23, 1909 FOREIGN PATENTS 504,585 Belgium July 31,1951 OTHER REFERENCES Marine Eng, vol. 58, No. 6, June 1953, pages69-73.

1. AN ARTICLE FOR USE IN THE CATHODIC PROTECTION OF METAL SURFACESIMMERSED IN AN ELECTROLYTE, COMPRISING A GALVANIC ANODE ASSEMBLYINCLUDING A GALVANIC ANODE BODY, A METAL CORE BONDED INTO SAID BODY, ANDA CLOSELY FITTING, ELECTRICALLY INSULATING COVERING SURROUNDING SAIDANODE BODY, SAID COVERING BEING A LAMINATED COVERING HAVING AN INNERLAYER AND AN OUTER LAYER, SAID COVERING BEING SOLUBLE IN SAIDELECTROLYTE, THE INNER LAYER BEING SUBSTANTIALLY LESS SOLUBLE IN THEELECTROLYTE THAN IS THE OUTER LAYER, THE OUTER LAYER ENTIRELY COVERINGSAID ANODE BODY.